Cloud patterns have long been read as short-range weather indicators, but more recently they have begun to be recognized as valuable long-term climatic signals. But their messages are far from clear, and little is known about the roles that clouds are likely to play in shaping future conditions on earth. Will they turn out to be agents of warming, veiling us in an ever-thickening blanket of emissions, or will they end up saving the day by reflecting more sunlight back into space? These, it turns out, are far from simple questions, and cloud behaviour continues to be the major impediment to understanding future climates, since a change in any aspect of clouds, such as their type, location, water content, longevity, altitude, or particle size, changes the way in which they might serve to warm or cool the earth. Clouds are truly the wild cards in our game of rethinking nature.

The Ozone ‘Hole’
Among the many barriers to effective action on climate change is the fact that we cannot actually see climate change; we can see some of its effects on the ground, but not its ultimate causes. Increases in atmospheric greenhouse gases occur invisibly, and though they can be measured and plotted on fearsome looking graphs, the fact of their invisibility constitutes a persistent conceptual block. If the sky somehow changed colour as a result of increased CO2, there wouldn’t be a problem today: emissions would have been halted long ago under a hastily-drafted international Clean Air Act. But since the sky looks more-or-less the same no matter what we pump into it, what is needed is some kind of convincing visual representation of atmospheric change, along the lines of the brilliantly coloured graphics that showed the ever-widening ‘hole’ in the ozone layer above Antarctica. First published in 1985 by NASA’s Scientific Visualization Studio at the Goddard Space Flight Center, these false-colour graphics succeeded in visualizing an otherwise invisible process, and in such a striking and memorable manner that they alerted the world to a looming environmental threat.  The world saw, took action, and the crisis was averted.

Fig. 1: The fluctuating ‘ozone hole’ over Antarctica: these are among the most effective scientific visualizations ever made (Credit: NASA/ Goddard Space Flight Center Scientific Visualization Studio).

Yet climate scientists have so far failed to find an ‘ozone hole’ equivalent for visualizing global warming, and what we have ended up with instead is a mess of competing visual narratives, the familiar roll-call of melting ice-caps, shrinking rivers, encroaching deserts, and rising waters. But perhaps we are looking down when we need to be looking up: rethinking nature in the light of climate change by reimagining and revisualizing the changing skies above us.

Clouds: The Height of Ambiguity
As is so often the case with climate science, current cloud research is producing apparently contradictory results. For instance, there are many climate scientists who believe that continued surface warming will cause increased water vapour to rise from the oceans, leading to an overall increase in cloud formation; while others maintain that an increase in the water vapour content of our atmosphere could see large convective cumuliform clouds building up and raining out far quicker than they do at present (particularly in warmer latitudes), thereby leading to a net decrease in the earth’s total cloud cover. And not only is it unclear which of these outcomes is the most likely, their long-term influences are scarcely understood, either. Even if, for the sake of argument, it’s assumed that overall cloud cover will increase as the surface of our planet warms, it is unclear what kind of clouds (and thus what kind of feedback mechanisms) are likely to predominate.

     For instance, high, thin clouds, such as cirrostratus, tend to have an overall warming effect, as they admit shortwave solar radiation in from above, while bouncing longwave back-radiation (reflected from the sunlit ground) back down to earth. Any increase in thin cloud cover would therefore add another warming mechanism to our climate. In contrast, however, bright, dense cumulus clouds serve to cool the earth by reflecting incoming sunlight back into space by day. So, in theory, an increase in high, thin clouds would amplify the global warming effect, while an increase in low, dense, puffy clouds would have a contrary cooling effect — which is why cloud-whitening has recently been advanced as a geo-engineering idea for mitigating climate change, with salt water to be sprayed from thousands of ships in order to create fleets of dense white clouds sailing over the oceans.

The Uncertain Effects of Contrails
But in reality the atmosphere is never that simple, and clouds have an interesting habit of behaving in unpredictable ways. For example, after the terrorist attacks of 11 September 2001, all commercial flights in the United States were grounded for several days, leaving the skies contrail-free for the first time in decades. The result, according to a comparison of nationwide temperature records, was slightly warmer days and slightly cooler nights than were usual for that time of year, the normal night/day temperature range having increased by 1.1° C. According to the climate scientists who worked on the data, this was probably due to additional sunlight reaching the surface by day, and additional radiation escaping at night through the unusually cloudless skies.*  At first sight this might seem counter-intuitive, for surely the kind of cirriform clouds created by the spreading of aircraft contrails are straightforward warming clouds, the kind that allow sunlight through, while bouncing back-radiation down to the lower atmosphere? Surely an absence of contrails ought to have an overall cooling effect?

    But contrails are a lot more complicated than that, because when they are in their initial, water droplet, stage they are denser than natural cirrus clouds, since they are created from two distinct sources of vapour: the moisture emitted by the aircraft’s exhaust, and the moisture already in the atmosphere, all of which is condensed into a turbulent mixture of large water droplets and ice crystals, seeded on the solid particulates present in the exhaust plume. At first, this young contrail behaves more like a fluffy low-level cloud, reflecting sunlight back into space, and exerting a short-term localized cooling effect. But if persistent contrails start to spread, they thin out into cirriform cloud layers, which can often cover large areas of sky. Their overall effect then reverts to a warming one, consistent with the known behaviour of natural cirriform clouds.

Fig. 2: Contrails over the southeastern United States, 29 January 2004 (Credit: NASA).

The picture is complicated yet further by the time of day that the contrails form and spread. If contrails spread during the early morning or late evening, they can exercise a slight cooling effect, due to the angle at which sunlight is reflected off the ice crystals into the upper atmosphere. At night, by contrast, all clouds, including contrails, can only exert a warming effect, since there is no incoming sunlight to reflect into space. Any increase in night flights is therefore likely to raise temperatures on the ground; and that increase is already well underway. In fact, the projected warming effects associated with the rise in night flights are in the region of  +0.2-0.3° C per decade in the United States alone — and this figure does not include other warming effects of aviation, such as increased CO2 emissions.** Will airlines have to change the altitudes or times of day at which they fly in order to modify their contrail production (especially in the skies of the developing world, where flights are becoming increasingly prevalent)? Of course, contrail science is a new research area, and little about these man-made clouds is understood entirely; as David Travis, the atmospheric scientist who led the post-9/11 contrail research, has pointed out: ‘what we’ve shown is that contrails are capable of affecting temperatures. Which direction, in terms of net heating or cooling, is still up in the air.’

Noctilucent Clouds
Equally up in the air, albeit at a far greater distance, are noctilucent clouds (NLCs), the changing patterns of which have become apparent over the past two decades. First observed and named in the 1880s, NLCs were once the highest and rarest clouds of all, but not only do they now appear more often than before, they also shine brighter, and are observable from increasingly lower latitudes.*** According to one hypothesis, NLCs are formed from plumes of space shuttle exhaust jettisoned into the earth’s upper atmosphere, where neither water vapour nor dust nuclei are common natural occurrences, and therefore these clouds’ increased appearance is due to a proportionate increase in space shuttle traffic. Other research, meanwhile, points to the fact that extreme cold is needed to form icy clouds in environments as dry as the mesosphere, 50-80 kilometres above the earth’s surface, where temperatures as low as -130° C are normal.
 

 Fig. 3: Noctilucent clouds from space, 11 June 2007 (Credit: NASA/ Cloud Imaging and Particle Size Experiment data processing team at the University of Colorado Laboratory for Atmospheric and Space Physics).

Strange as it may seem, the increased concentrations of atmospheric greenhouse gases that have contributed to raising temperatures on earth are also serving to create colder conditions in the earth’s outer atmosphere. This is because greenhouse gases trap much of the longwave surface radiation that has started its return journey back out into space. With less thermal energy able to escape from the lower atmosphere, the upper atmosphere is growing correspondingly colder. So could the observed increase in noctilucent cloud formation be due to mesospheric cooling, the lesser-known counterpart to global warming; and might their increased brightness be due to larger ice crystals being formed from a high-altitude influx of water vapour from the warming layers below? After all, NLCs have only been in evidence since the 1880s, the heyday of the Industrial Revolution, so it is possible that they will turn out to be yet another anthropogenic phenomenon. If that is the case, the visible impact of human activity will have extended much further into our fragile atmosphere than we ever previously thought. We will have succeeded in spreading man-made clouds not just over the skies above us, but out towards the very fringes of space.

RETHINK The Skies
If we are serious about our need to rethink human relations with nature, and if we are still in need of dramatic visualizations of anthropogenic climate change, a good starting place might be to step outside and take a good long look at our suffering sky, a now largely man-made layer dense with emissions, scored by contrails, and lit by anomalous night-shining clouds, through which an endless stream of electromagnetic weather from surveillance satellites, text messages, radio signals and emergency frequencies vibrates, day and night, like an invisible aurora, keeping the machinery of civilization alive.

* David J. Travis et al, ‘Contrails reduce daily temperature range’, Nature 418 (2002), 601.

** Patrick Minnis et al, ‘Contrails, Cirrus Trends, and Climate’, Journal of Climate 17:8 (2004), 1671-85

*** Sourish Basu, ‘Clouds that Rival Auroras Now Bigger and Brighter’, Scientific American Online News, 10 July 2007